J. Comp. Chem. [LCQ]

Résumé:

Une description en principe exacte des excitations électroniques “double” (promotion de deux électrons des orbitales occupées dans l’état fondamental vers des orbitales de plus hautes énergies) en théorie de la fonctionnelle de la densité (DFT) est proposée. Ces dernières sont absentes des spectres de DFT dépendante du temps (TDDFT) standard du fait de l’approximation adiabatique (absence de mémoire dans les fonctionnelles usuelles). Dans ce travail, un nouveau formalisme basé sur les ensembles dits “N-centrés étendus” est utilisé, permettant ainsi une exactification des énergies orbitalaires Kohn—Sham dans la description du processus d’excitation. 

Abstract:

A recent work (arXiv:2401.04685) has merged N-centered ensembles of neutral and charged electronic ground states with ensembles of neutral ground and excited states, thus providing a general and in-principle exact (so-called extended N-centered) ensemble density functional theory of neutral and charged electronic excitations. This formalism made it possible to revisit the concept of density-functional derivative discontinuity, in the particular case of single excitations from the highest occupied Kohn–Sham (KS) molecular orbital, without invoking the usual “asymptotic behavior of the density” argument. In this work, we address a broader class of excitations, with a particular focus on double excitations. An exact implementation of the theory is presented for the two-electron Hubbard dimer model. A thorough comparison of the true physical ground- and excited-state electronic structures with that of the fictitious ensemble density-functional KS system is also presented. Depending on the choice of the density-functional ensemble as well as the asymmetry of the dimer and the correlation strength, an inversion of states can be observed. In some other cases, the strong mixture of KS states within the true physical system makes the assignment “single excitation” or “double excitation” irrelevant.

Reference:

Extended N-centered ensemble density functional theory of double electronic excitations

Filip Cernatic and Emmanuel Fromager

Journal of Computationnel Chemistry, First published: 03 May 2024 – DOI: https://doi.org/10.1002/jcc.27387

Contact:

E. Fromager, team LCQ, Institut de Chimie (UMR 7177).